A Model for the Dust Envelope of the Silicate Carbon Star Iras 09425-6040

Abstract

IRAS 09425-6040 (I09425) is a silicate carbon star with conspicuous crystalline silicate and water-ice features and emission excesses in the far-infrared and millimeter (mm) wavelength ranges. To understand properties of the dust envelope of I09425, we propose a physical model based on the observations and known properties of asymptotic giant branch stars and dust. We perform radiative transfer model calculations using multiple dust shells and disks with various dust species. We compare the model results with the observed spectral energy distribution (SED) acquired with different telescopes. We find that the physical model for I09425 using multiple shells of carbon and silicate dust and multiple disks of amorphous and crystalline silicates reproduces the observed SED fairly well. This object looks to have detached cold O-rich (silicate and water-ice) dust shells, which could be remnants of the recent chemical transition from O to C and an inner C-rich dust shell. A long-lived thin disk of very large silicate grains can reproduce the emission excess in the mm wavelength band and a recently formed thick disk of crystalline silicates can reproduce the prominent emission features in the spectral range 8-45 μm. The highly crystallized silicates could be recently formed by high temperature annealing due to the last O-rich superwind just before the chemical transition of the central star. I09425 could be a rare object that has the remnants of past O-rich stellar winds in the outer shells as well as in the circumbinary disks.Key words: circumstellar matter -dust, extinction -infrared: stars -radiative transfer -stars: AGB and post-AGB INTODUCTIONBased on the chemistry of the photosphere and/or the circumstellar envelope, asymptotic giant branch (AGB) stars are generally classified as O-rich (M-type) or C-rich (C-type). Carbon stars are generally believed to be the evolutionary successors of O-rich AGB stars because an O-rich AGB star may become a carbon star when the star goes through C dredge-up processes due to thermal pulses (Iben 1981;Chan & Kwok 1990).As an AGB star evolves, a thermal pulse due to an internal helium shell flash has been hypothesized as a major cause of an episode of greatly enhanced mass-loss, commonly referred to as a superwind phase (Wood 1990). One thermal pulse every 10 4 -10 5 years that endures for a few hundred years is expected (Iben & Renzini 1983;Vassiliadis & Wood 1993). The chemical transition from O to C and superwind, which is induced by a thermal pulse, is known to make major effects on the dust envelopes around AGB stars (e.g., Groenewegen et al. 1995;Suh 2014).Silicate carbon stars show the characteristics of a carbon star and circumstellar silicate dust features. They were discovered by IRAS observations (Little-Marenin 1986; Willems & de Jong 1986). Willems & de Jong (1986) and Chan & Kwok (1991) suggested that silicate carbon stars are objects in transition from O-rich AGB stars to carbon stars. Many of the silicate carbon stars are fairly well fitted with a simple deta...

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“…The formation of both carbon rich and oxygen rich dust could be due to a chemical gradient in the wind as the stars evolve from oxygen rich to carbon rich after undergoing C dredge-up processes due to a recent AGB thermal pulse. Suh (2016) successfully modeled the dust envelope around IRAS 09425-6040 with an outer oxygen rich shell and an inner carbon rich shell, validating this hypothesis. Similarly, the carbon-rich planetary nebula BD +30 • 3639 exhibits spatially separated carbon-rich polycyclic aromatic hydrocarbons and oxygen-rich silicate dust (Guzman-Ramirez et al 2015).…”

“…The amount of crystalline grain material compared to amorphous grain material is generally low, ∼10-15%, and is dominant only in rare cases. Crystalline material is also generally only detected in stars that have experienced high mass loss rates (above 10 −5 M yr −1 ) (Cami et al 1998;Sylvester et al 1999;Sogawa & Kozasa 1999;Suh 2002). However, both crystalline and amorphous grains have been detected simultaneously in stellar outflows, both in the present study as well as in others.…”

“…This dual formation of carbon and oxygen rich minerals has been observed in several classical novae, namely V1280 Sco (Sakon et al 2016), V705 Cas , V842 Cen (Smith et al 1994), and QV Vul (Gehrz et al 1992) as well as IRAS 09425-6040, a carbon AGB star which shows circumstellar silicate dust features (Suh 2016). The formation of both carbon rich and oxygen rich dust could be due to a chemical gradient in the wind as the stars evolve from oxygen rich to carbon rich after undergoing C dredge-up processes due to a recent AGB thermal pulse.…”

“…The interpulse time exceeds the expansion time for circumstellar shells and as expected nearly all normal AGB stars have shells of the same composition as the star. Carbon rich AGB stars are expected to have circumstellar shells dominated by amorphous carbon or graphite grains with some silicon carbide (SiC) possibly present (Suh 2000;Speck et al 2005Speck et al , 2009). Amorphous carbon does not have prominent IR features but contributes to the dust continuum emission, however, graphitic carbon and silicon carbide have emission features at 11.53 µm and in the 10-13 µm region, respectively.…”

“…The formation of both carbon rich and oxygen rich dust could be due to a chemical gradient in the wind as the stars evolve from oxygen rich to carbon rich after undergoing C dredge-up processes due to a recent AGB thermal pulse. Suh (2016) AGB binary HR 4049 is a peculiar example of a depleted oxygen rich star with a featureless mid-IR spectrum possibly resulting from amorphous carbon masking the spectral features from silicates (Acke et al 2013). While it is possible we may be observing the stars in transition from oxygen rich to carbon rich, it would require that all of these stars result from a narrow range of masses that terminate AGB evolution just as the carbon exceeds the oxygen abundance.…”